Method and apparatus for ultrasonic beamforming using golay-coded excitation

ABSTRACT

A method and apparatus for improving the SNR in medical ultrasound imaging utilize Golay-coded excitation of the transducer array. A Golay pair is a pair of binary (+1,-1) sequences with the property that the sum of the autocorrelations of the two sequences is a Kronecker delta function. This translates into two important advantages over codes in general: (1) Golay codes have no range sidelobes, and (2) Golay codes can be transmitted using only a bipolar pulser versus a more expensive digital-to-analog converter. Degradation of the Golay code is avoided by employing multiple focal zones, where the Golay code is used only in the deepest focal zones in order to minimize dynamic focusing effects, and by employing two consecutive transmits on each beam to minimize tissue motion between the two code sequences.

FIELD OF THE INVENTION

This invention generally relates to ultrasound imaging systems and, moreparticularly, to methods and apparatus for increasing thesignal-to-noise ratio (SNR) in medical ultrasound imaging.

BACKGROUND OF THE INVENTION

Conventional ultrasound imaging systems comprise an array of ultrasonictransducer elements which transmit an ultrasound beam and then receive areflected beam from the object being studied. This operation comprises aseries of measurements in which a focused ultrasonic wave istransmitted, the system switches to receive mode after a short timeinterval, and the reflected ultrasonic wave is received, beamformed andprocessed for display. Transmission and reception are typically focusedin the same direction during each measurement to acquire data from aseries of points along an acoustic beam or scan line. The receiver isdynamically focused at a succession of ranges along the scan line as thereflected ultrasonic waves are received.

For ultrasound imaging, the array typically has a multiplicity oftransducer elements arranged in one or more rows and driven withseparate voltages. By selecting the time delay (or phase) and amplitudeof the applied voltages, the individual transducer elements in a givenrow can be controlled to produce ultrasonic waves which combine to forma net ultrasonic wave that travels along a preferred vector directionand is focused at a selected point along the beam. The beamformingparameters of each of the firings may be varied to provide a change inmaximum focus or otherwise change the content of the received data foreach firing, e.g., by transmitting successive beams along the same scanline with the focal point of each beam being shifted relative to thefocal point of the previous beam. In the case of a steered array, bychanging the time delays and amplitudes of the applied voltages, thebeam with its focal point can be moved in a plane to scan the object. Inthe case of a linear array, a focused beam directed normal to the arrayis scanned across the object by translating the aperture across thearray from one firing to the next.

The same principles apply when the transducer probe is employed toreceive the reflected sound in a receive mode. The voltages produced atthe receiving transducer elements are summed so that the net signal isindicative of the ultrasound reflected from a single focal point in theobject. As with the transmission mode, this focused reception of theultrasonic energy is achieved by imparting separate time delays (and/orphase shifts) and gains to the signal from each receiving transducerelement.

An ultrasound image is composed of multiple image scan lines. A singlescan line (or small localized group of scan lines) is acquired bytransmitting focused ultrasound energy at a point in the region ofinterest, and then receiving the reflected energy over time. The focusedtransmit energy is referred to as a transmit beam. During the time aftertransmit, one or more receive beamformers coherently sum the energyreceived by each channel, with dynamically changing phase rotation ordelays, to produce peak sensitivity along the desired scan lines atranges proportional to the elapsed time. The resulting focusedsensitivity pattern is referred to as a receive beam. Resolution of ascan line is a result of the directivity of the associated transmit andreceive beam pair.

The output signals of the beamformer channels are coherently summed toform a respective pixel intensity value for each sample volume in theobject region or volume of interest. These pixel intensity values arelog-compressed, scan-converted and then displayed as an image of theanatomy being scanned.

In medical ultrasound imaging systems of the type described hereinabove,it is desirable to optimize the SNR. Additional SNR can be used toobtain increased penetration at a given imaging frequency or to improveresolution by facilitating ultrasonic imaging at a higher frequency.

The use of Golay code in ultrasound is well known in the area ofnon-destructive evaluation (NDE) using single-element fixed-focustransducers to inspect inanimate objects. Golay code is also known inthe medical ultrasound imaging community. However, the use of Golay codein an ultrasound imaging system of the type described above has beendismissed because dynamic focusing, tissue motion (effects not presentin NDE) and nonlinear propagation effects are thought to causeunacceptable code degradation with corresponding range degradation.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for improving the SNR inmedical ultrasound imaging by using Golay-coded excitation of thetransducer array without unacceptable degradation of the Golay code.Code degradation is avoided by employing multiple focal zones, where theGolay code is used only in the deepest focal zones in order to minimizedynamic focusing effects and nonlinear propagation effects. Golay codeis not used in the shallow zones where there is adequate SNR. Codedegradation due to tissue motion during the interval between twotransmit firings has been found to be acceptable.

The SNR is improved by transmitting a pair of Golay-encoded basesequences consecutively on each beam at the same focal position and thendecoding the beamsummed data. The imaging depth is divided into multiplefocal zones, with coded excitation used only for the deepest focalzone(s). The deepest zones have the largest f-numbers, which result inthe least code distortion due to dynamic focusing. In addition, thedeepest zones have a need for SNR improvement.

A pair of Golay-encoded base sequences are formed by convolving a basesequence with a Golay code pair after oversampling. A Golay code pair isa pair of binary (+1, -1) sequences with the property that the sum ofthe autocorrelations of the two sequences is a Kronecker delta function.An oversampled Golay sequence is the Golay sequence with zeroes inbetween each +1 and -1, the number of zeroes being greater than or equalto one less than the length of the base sequence.

The aforementioned property of Golay code pairs translates into twoimportant advantages over codes in general: (1) Golay codes have norange sidelobes, and (2) Golay codes can be transmitted using only abipolar pulser versus a more expensive digital-to-analog converter.

By transmitting two sequences of pulses that are polarity-encodedaccording to a Golay pair, the correlation of each of the receivedbeamsum signals with its corresponding oversampled Golay sequence andthe summation of those correlations enables an increase in the SNR withvirtually no degradation in image resolution or contrast. In practice,range sidelobes do occur due to code distortion, but they tend to bebelow the noise floor (which can be quite high in the deep focal zones)and do not adversely affect image quality.

Nonlinear propagation effects distort the code at high signal amplitude.However, the signal amplitude is low in deep zones. Although a nonlinearsignal generated in shallow zones continues to propagate, such signalshave higher frequencies, i.e., harmonics or multiples of the fundamentalfrequency, so they attenuate at a higher rate than does the fundamentalfrequency. At deeper focal zones, nonlinear signals generated earlierhave substantially died out. Thus, by using the Golay code only in deepzones--where nonlinear propagation effects are small--code distortion isavoided.

Tissue motion that occurs in the interval between transmission of thetwo sequences of the Golay pair also causes code distortion whichincreases the range sidelobes. By transmitting the second sequence assoon as the echoes from the first sequence have been completelyreceived, duration of the interval between the two transmits can beminimized. Minimization of the interval between transmits in turnminimizes the motion-induced code distortion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the major functional subsystems withina conventional real-time ultrasound imaging system.

FIG. 2 is a block diagram showing further details of the pulsing andreceiving subsystems incorporated in the system depicted in FIG. 1.

FIG. 3 is block diagram of an ultrasonic imaging system usingGolay-coded excitation of transducer elements and decoding of thereceive waveform in accordance with the present invention.

FIGS. 4-8 are pulse diagrams showing the base sequence (FIG. 4), theoversampled Golay sequences (FIGS. 5 and 6), and the Golay-encoded basesequences (FIGS. 7 and 8) in accordance with one preferred embodiment ofthe invention.

FIG. 9 is a block diagram showing the arrangement for Golay-encodedexcitation of a single transducer element in accordance with the presentinvention.

FIG. 10 is a graph of the spectrum magnitude versus frequency for thebase sequence shown in FIG. 4 and for the base sequence [-1,+1,+1,-1].

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a conventional ultrasonic imaging systemincorporating a transducer array 10 comprised of a plurality ofseparately driven transducer elements 12, each of which produces a burstof ultrasonic energy when energized by a pulsed waveform produced by atransmitter 14. The ultrasonic energy reflected back to transducer array10 from the object under study is converted to an electrical signal byeach receiving transducer element 12 and applied separately to areceiver 16 through a set of transmit/receive (T/R) switches 18. The T/Rswitches 18 are typically diodes which protect the receive electronicsfrom the high voltages generated by the transmit electronics. Thetransmit signal causes the diodes to shut off or limit the signal to thereceiver. Transmitter 14 and receiver 16 are operated under control of abeamformer controller 20 responsive to commands by a human operator. Acomplete scan is performed by acquiring a series of echoes in whichtransmitter 14 is gated ON momentarily to energize each transducerelement 12, and the subsequent echo signals produced by each transducerelement 12 are applied to receiver 16. A transducer element may beactuated to begin reception while another transducer element is stilltransmitting. Receiver 16 combines the separate echo signals from eachtransducer element to produce a single echo signal which is used toproduce a line in an image on a display monitor 28.

Under the direction of beamformer controller 20, transmitter 14 drivestransducer array 10 such that the ultrasonic energy is transmitted as adirected focused beam. To accomplish this, respective time delays areimparted to a multiplicity of pulsers 28, shown in FIG. 2. Each pulseris coupled to a respective transducer element via T/R switches 18. Thetransmit focus time delays are preferably read from a look-up table 30.By appropriately adjusting the transmit focus time delays in aconventional manner, the ultrasonic beam can be directed and focused ata point.

The echo signals produced by each burst of ultrasonic energy reflectfrom objects located at successive ranges along the ultrasonic beam. Theecho signals are sensed separately by each transducer element 12, shownin FIG. 1, and a sample of the echo signal magnitude at a particularpoint in time represents the amount of reflection occurring at aspecific range. Due to differences in the propagation paths between areflecting point and each transducer element 12, however, these echosignals will not be detected simultaneously and their amplitudes willnot be equal. Receiver 16 amplifies the separate echo signals, impartsthe proper time delay to each, and sums them to provide a single echosignal which accurately indicates the total ultrasonic energy reflectedfrom a specific point located at a particular range along the ultrasonicbeam.

Under the direction of beamformer controller 20, as shown in FIG. 1,receiver 16 tracks the direction of the transmitted beam and acquiresthe echo signals at a succession of ranges. Each transmission of anultrasonic pulse waveform results in acquisition of data whichrepresents the amount of sonic energy reflected from correspondingranges along the ultrasonic beam. To accomplish this, respective receivefocus time delays are imparted to a multiplicity of receive channels 32of receiver 16, as shown in FIG. 2. Each receive channel is couplted toa respective transducer element via T/R switches 18. The receive focustime delays are computed in real-time using specialized hardware 34 orread from a look-up table. The receive channels include circuitry (notshown) for apodizing and filtering the received pulses. The time-delayedreceive signals are then summed in a receive summer 36.

A signal processor or detector 22 converts the summed received signal todisplay data. In the B-mode (grey-scale), this constitutes the signalenvelope with some additional processing, such as edge enhancement andlogarithmic compression. A scan converter 24, shown in FIG. 1, receivesthe display data from detector 22 and converts the data into the desiredimage for display. In particular, scan converter 24 converts theacoustic image data from polar coordinate (R-θ) sector format orCartesian coordinate linear array to appropriately scaled Cartesiancoordinate display pixel data at the video rate. These scan-convertedacoustic data are supplied for display to display monitor 26, whichimages the time-varying amplitude of the signal envelope as a greyscale.

FIG. 3 is a block diagram of a medical ultrasound imaging system inaccordance with the present invention, with the beamformer controller,such as shown in FIG. 3, omitted for simplicity of illustration. Theimaging system operates in conventional manner when imaging shallowtransmit focal zones (which generally have adequate SNR). However, fordeep transmit focal zones (which generally have inadequate SNR) thesystem uses Golay-encoded excitation.

During each firing of ultrasonic energy, bipolar pulsers 28' are excitedby a Golay-encoded base sequence output signal from a transmit memory 38or from specialized hardware. In response to the Golay-encoded basesequence from transmit memory 38 and transmit focus delays supplied froma look-up table 30, the pursers produce Golay-encoded pulse sequences tothe respective transducer elements 12 (FIG. 1), making up the transmitaperture. FIG. 9 shows one such base sequence stored in transmit memory38 for driving a transducer element 12. The +1 and -1 elements of eachGolay-encoded base sequence are transformed into pulses of oppositephase by the bipolar pulsers. A pair of Golay-encoded base sequences aretransmitted consecutively on each beam, i.e., during first and secondfirings having the same focal position.

For each firing, the echo signals resulting from the focused beamreceived at the transducer elements are transduced into electricalsignals by the transducer elements making up the receive aperture. Thesereceived signals are amplified and time-delayed in receive channels 32in accordance with the receive focus time delays computed in real-timeby a processor 34 or, alternatively, supplied from a look-up table (notshown). The amplified and delayed signals are summed by receive beamsummer 36.

The summed receive signal is decoded by a Golay decoder 40. For eachfiring, decoding is performed using the oversampled Golay sequencecorresponding to the Golay-encoded base sequence employed duringtransmission. The oversampled Golay sequences are stored in a memory 42and are supplied to decoder 40 at the appropriate time.

In accordance with a preferred embodiment of the invention, Golaydecoder 40 comprises a finite impulse response (FIR) filter 44 and abuffer memory 46 having an input coupled to the output of the FIRfilter. For the first firing, a first set of filter taps are read out ofGolay sequence memory 42 to FIR filter 44. The beamsummed signalproduced following the first firing is then filtered and stored inbuffer memory 46. For the second firing, a second set of filter taps areread out of Golay sequence memory 42 to FIR filter 44. The beamsummedsignal produced following the second firing is then filtered andsupplied to buffer memory 46, where the filtered beamsummed signal fromthe second firing is added to the filtered beamsummed signal from thefirst firing.

FIGS. 4-8 illustrate formation of the transmit (Golay-encoded) basesequences from the convolution of the base sequence with the respectiveone of a pair of oversampled Golay sequences. The base sequence isdesigned to optimize the resulting ultrasonic pulse shape and spectralenergy. In the example depicted in FIG. 4, the base sequence is asequence of pulses having the following polarities:[+1,+1,+1,+1,-1,-1,-1,-1]. For the first firing, the base sequence isconvolved with oversampled Golay sequence A (see FIG. 5) correspondingto Golay code [+1,+1]. The resulting Golay-encoded base sequence A* isshown in FIG. 6. For the second firing, the base sequence is convolvedwith oversampled Golay sequence B (see FIG. 7) corresponding to Golaycode [+1,-1]. The resulting Golay-encoded base sequence B* is shown inFIG. 8. The Golay-encoded base sequences are precomputed and stored intransmit memory 38, shown in FIG. 3. The transmit sequence, afterexciting the transducer element, results in a sequence of ultrasonicpulses with polarity given by a Golay sequence for each firing.

The base sequence can be optimized to ensure that maximum energy passesthrough the transducer passband. For example, FIG. 10 shows the spectrummagnitude as a function of frequency for two base sequences:[+1,+1,+1,+1,-1,-1,-1,-1] and [-1,+1,+1,-1]. As seen in FIG. 10,assuming a sampling rate of 40 MHz, the former sequence produces atransmitted pulse centered at 5 MHz and the latter sequence produces atransmitted pulse centered at 10 MHz. The appropriate base sequence canbe selected depending on the operating characteristics of the transducerand the desired point spread function.

The transmitted waveform is generated by exciting each transducerelement 12 with a sequence of regularly spaced bipolar pulses, as shownin FIG. 9. This pulse sequence is specified by a sequence of +1's and-1's stored in transmit memory 38 and provided to bipolar pulser 28'.Although FIG. 9 depicts a transmit memory storing only eight samples, inpractice the transmit memory will store 64, 128 or more samples read outat a sampling rate of, e.g., 40 MHz. For a Golay code pair [+1,+1] and[+1, -1] and a base sequence of [-1,+1,+1,-1], the followingGolay-encoded base sequence A* would be stored in the transmit memoryfor the first firing: [-1,+1,+1,-1,-1,+1,+1,-1]. For the second firing,the following Golay-encoded base sequence B* would be stored in thetransmit memory: [-1,+1,+1,-1,+1,-1,-1,+1].

For each beam in a deep transmit focal zone, sequence A* is transmittedfirst. Then the echo signal from the first firing is digitized,beamsummed, filtered and stored in buffer memory 46 (see FIG. 3).Subsequently, sequence B* is transmitted and its echo signal issimilarly processed. The two beamsummed signals are filtered tocorrelate each signal with its respective oversampled Golay sequence (Aand B in FIGS. 5 and 7, respectively). FIR filter 44, shown in FIG. 3,performs the correlation: ##EQU1## where * denotes convolution and theoverbar denotes conjugation (if x and y are complex). The results of thecorrelations are summed in buffer memory 46 to form the decoded signal,which is supplied to the B-mode processor (not shown) for furtherprocessing. Except for improved SNR, the decoded Golay pulse isvirtually the same as that obtained by transmitting the base sequenceinstead of the Golay-encoded base sequence.

A major advantage of the Golay code lies in its use of a bipolar pulserfor code transmission versus the more expensive digital-to-analogconverter that is required to transmit other codes such as the apodizedchirp. In addition, the Golay code theoretically has no range lobes,which is not true of any other code.

The imaging system of the invention can also operate by demodulating theRF echo signals to baseband and down-sampling before or after thebeamsum. In this instance, the oversampled Golay sequences A and B thatare stored for correlation would also be demodulated to baseband anddownsampled.

While only certain preferred features of the invention have beenillustrated and described herein, many modifications and changes will beapparent to those skilled in the art. For example, the invention is notlimited to using biphase codes; polyphase codes can alternatively beused. In addition, it will be apparent that Golay coding can beperformed on separate receive subapertures to reduce the effects ofdynamic focusing. For example, a receive aperture can be divided intotwo or more subapertures for a single transmit event. The subaperturescan be different for the two transmit events provided that the overallreceive aperture is the same. For each transmit event the receivesignals are beamformed for each subaperture, the beamformed signals forthe respective subapertures are filtered, and the filtered signals ofthe respective subapertures are summed. It is, therefore, to beunderstood that the appended claims are intended to cover all suchmodifications and changes as fall within the true spirit of theinvention.

We claim:
 1. A system for imaging ultrasound scatterers, comprising:anultrasound transducer array for transmitting ultrasound waves anddetecting ultrasound echoes reflected by said ultrasound scatterers,said transducer array comprising a multiplicity of transducer elements;transmit means coupled to said transducer array for pulsing a firstplurality of transducer elements which form a transmit aperture with afirst Golay-encoded pulse sequence during a first transmit event andwith a second Golay-encoded pulse sequence during a second transmitevent, said first and second Golay-encoded pulse sequences beingrespectively derived from convolution of a base sequence with first andsecond oversampled Golay sequences of a pair of Golay sequences, saidpair of Golay sequences having the property that the sum of theautocorrelations of said first and second Golay sequences is a Kroneckerdelta function; receive means coupled to said transducer array forreceiving a first set of signals from a second plurality of transducerelements which form a receive aperture subsequent to said first transmitevent and a second set of signals from said second plurality oftransducer elements subsequent to said second transmit event;beamforming means for forming first and second beamsummed receivesignals from said first and second sets of signals respectively; meansfor filtering said first beamsummed receive signal as a function of saidfirst oversampled Golay sequence to form a first filtered signal and forfiltering said second beamsummed receive signal as a function of saidsecond oversampled Golay sequence to form a second filtered signal;means for summing said first and second filtered signals to form adecoded signal; and means for displaying an image which is a function ofsaid decoded signal.
 2. The system as defined in claim 1, wherein saidselected transducer elements are focused at the same focal positionduring said first and second transmit events.
 3. The system as definedin claim 1, wherein said transmit means comprises a plurality of bipolarpulsers respectively coupled to said first plurality of transducerelements and means for providing said first and second Golay-encodedbase sequences to each of said plurality of bipolar pulsers.
 4. Thesystem as defined in claim 3, wherein said means for providing saidfirst and second Golay-encoded base sequences comprises first memorymeans for storing said first and second Golay-encoded pulse sequences.5. The system as defined in claim 4, wherein said means for providingsaid first and second Golay-encoded base sequences further comprisessecond memory means for storing said transmit focus time delays fordelaying activation of said bipolar pulsers.
 6. The system as defined inclaim 1, wherein said filtering means comprises:means for correlatingsaid first beamsummed receive signal with said first oversampled Golaysequence and said second beamsummed receive signal with said secondoversampled Golay sequence to form said first and second filteredsignals respectively.
 7. The system as defined in claim 1, wherein saidfiltering means comprises an FIR filter and memory means for providingfirst and second sets of filter taps to said FIR filter, said first andsecond sets of filter taps corresponding respectively to said first andsecond oversampled Golay sequences.
 8. A method for imaging ultrasoundscatterers, comprising the steps of:deriving first and secondGolay-encoded base sequences from convolution of a base sequence withfirst and second oversampled Golay sequences respectively, the sum ofthe autocorrelations of said first and second Golay sequences being aKronecker delta function; driving a first set of transducer elementsforming a transmit aperture in a transducer array with said firstGolay-encoded pulse sequence during a first transmit event; receiving afirst set of echo signals from a second set of transducer elementsforming a receive aperture in the transducer array subsequent to saidfirst transmit event; forming a first beamsummed receive signal fromsaid first set of echo signals; filtering said first beamsummed receivesignal as a function of said first oversampled Golay sequence to form afirst filtered signal; driving said first set of transducer elementswith said second Golay-encoded pulse sequence during a second transmitevent subsequent to said first transmit event; receiving a second set ofecho signals from said second set of transducer elements subsequent tosaid second transmit event; forming a second beamsummed receive signalfrom said second set of echo signals; filtering said second beamsummedreceive signal as a function of said second oversampled Golay sequenceto form a second filtered signal; summing said first and second filteredsignals to form a decoded signal; and displaying an image as a functionof said decoded signal.
 9. The method as defined in claim 8, includingthe step of focusing said first and second sets of transducer elementsat the same focal position during said first and second transmit events.10. The method as defined in claim 8, wherein the step of filteringcomprises the steps of:correlating said first beamsummed receive signalwith said first oversampled Golay sequence and correlating said secondbeamsummed receive signal with said second oversampled Golay sequence toform said first and second filtered signals respectively.
 11. A methodfor imaging ultrasound scatterers, comprising the steps of:generatingfirst and second Golay-encoded pulse sequences; transmitting, from anarray of transducer elements during a first transmit event, a firstfocused ultrasound beam having a focal depth located in a first transmitfocal zone and derived from pulsing each one of a first multiplicity oftransducer elements with said first Golay-encoded pulse sequence;receiving, subsequent to said first transmit event, a first set of echosignals from a second multiplicity of transducer elements forming areceive aperture in the transducer array; forming a first beamsummedreceive signal from said first set of echo signals; filtering said firstbeamsummed receive signal to form a first filtered signal; transmitting,from an array of transducer elements during a second transmit event, asecond focused ultrasound beam having a focal depth located in saidfirst transmit focal zone and derived from pulsing each one of saidfirst multiplicity of transducer elements with said second Golay-encodedpulse sequence; receiving, subsequent to said second transmit event, asecond set of echo signals from said second multiplicity of transducerelements; forming a second beamsummed receive signal from said secondset of echo signals; filtering said second beamsummed receive signal toform a second filtered signal; summing said first and second filteredsignals to form a decoded signal; and displaying an image as a functionof said decoded signal.
 12. The method as defined in claim 11, whereinsaid first and second focused ultrasound beams are focused at a commonfocal position.
 13. The method as defined in claim 11, wherein the stepof generating first and second Golay-encoded pulse sequences comprisesthe steps of:convolving a base sequence with a first oversampled Golaysequence of a pair of Golay sequences wherein the sum of theautocorrelations of said first and second Golay sequences is a Kroneckerdelta function; and convolving said base sequence with a secondoversampled Golay sequence of said pair of Golay sequences.
 14. Themethod as defined in claim 13, wherein the step of filtering said firstbeamsummed receive signal comprises correlating said first beamsummedreceive signal with said first oversampled Golay code and wherein thestep of filtering said second beamsummed signal comprises correlatingsaid second beamsummed receive signal with said second oversampled Golaycode.
 15. The method as defined in claim 11, further comprising the stepof transmitting, during a third transmit event, a third focusedultrasound beam having a focal depth located in a second transmit focalzone situated at a shallower depth than the depth of said first transmitfocal zone and derived from pulsing each one of a third multiplicity oftransducer elements with a sequence of pulses which are notGolay-encoded.
 16. The method as defined in claim 11, wherein the secondtransmit event is the next transmit event occuring after the firsttransmit event.
 17. A method for imaging ultrasound scatterers,comprising the steps of:generating first and second Golay-encoded pulsesequences; transmitting, from an array of transducer elements during afirst transmit event, a first focused ultrasound beam having a focaldepth located in a first transmit focal zone and derived from pulsingeach one of a first multiplicity of transducer elements with said firstGolay-encoded pulse sequence; receiving, subsequent to said firsttransmit event, a first set of echo signals from a second multiplicityof transducer elements forming a first receive subaperture in thetransducer array; forming a first beamsummed receive signal from saidfirst set of echo signals; filtering said first beamsummed receivesignal to form a first filtered signal; receiving, subsequent to saidfirst transmit event, a second set of echo signals from a thirdmultiplicity of transducer elements forming a second receive subaperturein the transducer array; forming a second beamsummed receive signal fromsaid second set of echo signals; filtering said second beamsummedreceive signal to form a second filtered signal; summing said first andsecond filtered signals to form a first summed filtered signal;transmitting, from an array of transducer elements during a secondtransmit event, a second focused ultrasound beam having a focal depthlocated in said first transmit focal zone and derived from pulsing eachone of said first multiplicity of transducer elements with said secondGolay-encoded pulse sequence; receiving, subsequent to said secondtransmit event, a third set of echo signals from a fourth multiplicityof transducer elements forming a third receive subaperture in thetransducer array; forming a third beamsummed receive signal from saidthird set of echo signals; filtering said third beamsummed receivesignal to form a third filtered signal; receiving, subsequent to saidsecond transmit event, a fourth set of echo signals from a fifthmultiplicity of transducer elements forming a fourth receive subaperturein the transducer array; forming a fourth beamsummed receive signal fromsaid fourth set of echo signals; filtering said fourth beamsummedreceive signal to form a fourth filtered signal; summing said third andfourth filtered signals to form a second summed filtered signal; summingsaid first and second summed filtered signals to form a decoded signal;and displaying an image which is a function of said decoded signal. 18.The method as defined in claim 17, wherein said first and second focusedultrasound beams are focused at a common focal position.